The most important machine at the present time for processing plastic and polymeric materials is the single screw extruder. The term processing refers to one or more of the following operations: handling and conveying and pressurizing of particulate solids; melting or plasticating of solid materials; conveying, pressurizing or pumping of liquid or molten materials; mixing blending, dispersing and homogenizing the materials and various liquid or solid additives; devolatilizing the materials; bringing about any microscopic or macroscopic structural change in the materials by chemical reactions such as polymerization, cross-linking and foaming or by other means to modify, alter or improve some property. It is well recognized in the field, that the ordinary screw extruder for processing plastic and polymeric materials is generally modelled as a shallow stationary channel defined by the root of the screw and the flights and a fourth wall defined by the inner surface of the barrel which moves relative to the stationary channel, (see for example Z. Tadmor and I. Klein, "Engineering Principles of Plasticating Extrusion", Van Nostrand Reinhold Book Co., New York, 1970).
The relative rotational movement of the barrel and the extruder screw drags material both in particulate solids and viscous liquid form toward the discharge end of the barrel and toward a screw flight bringing about conveying, pressurization and pumping actions of solids and viscous liquids, and mixing dispersion and homogenization of the viscous liquids. Heat energy from the barrel together with frictional heat generation and viscous heat generation create a relatively thin film of melt on the barrel surface, which is dragged by the said relative motion toward a screw flight where it is scraped off bringing about an efficient melting and plasticating action.
Material on the root of the screw and flights of the screw cannot be scraped off, neither do these walls, which are stationary relative to the processed material, bring about any dragging action toward the discharge end of the barrel to facilitate conveying, pressurization or pumping of the solids and viscous material, nor do they facilitate in the mixing, dispersing and homogenization process. In the screw extruder, therefore, the single surface, that of the barrel, is the sole agent for processing material.